Short-term physiological indicators of N deficiency in phytoplankton: a unifying model

Nitrogen resupply to N-limited microalgae elicits several rapid physiological responses which have been used as indicators of N deficiency. These include a stimulation of dark respiration and dark carbon fixation. In addition, N resupply to some microalgae results in a transient suppression of photosynthetic carbon fixation. Long term, N enrichment results in an increase in the rate of photosynthesis and ultimately in the rate of growth. In this report these physiological indicators of N deficiency are documented as a function of steady-state growth rate in Selenastrum minutum Naeg. Collins (Chlorophyta). Based on previous biochemical and physiological studies, we suggest that these short-term responses are a reflection of redirection of photosynthetic carbon flow to the process of N assimilation. A unifying model is proposed that interrelates these responses to N resupply.